A belt for a belt type continuously variable transmission comprises plural elements supported between a pair of endless rings. The endless rings are formed by laminated thin plates. When the continuously variable transmission is running, a tensile force and a bending moment act on the belt which transmits the torque. As the endless rings have to bear these loads, a large tensile stress occurs on the outer circumferences of the endless rings.
JP2000-087214 published by the Japan Patent Office in 2000 proposes giving the surface of the thin plate which forms the endless ring a nitriding treatment for reducing this tensile stress so as to add a prestress in a compression direction. The prestress in the compression direction has the effect of eliminating the tensile stress which is produced when the belt is used. Therefore, in this prior art, the load borne by the endless rings when torque is transmitted can be reduced, and the fatigue strength of the belt can be improved.